Re: [PATCH v2 0/6] ARM: Add support for large kernel page (from 8K to 64K)

From: Arnd Bergmann <arnd@arndb.de>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 0/6] ARM: Add support for large kernel page (from 8K to 64K)
Date: Fri, 12 Jun 2020 11:23:11 +0200	[thread overview]
Message-ID: <CAK8P3a0KZzR0_8HWXYZRgq8Cg83M0s9GZa5wV+yz9KhvaSWEDw@mail.gmail.com> (raw)
In-Reply-To: <20200611162117.GY1551@shell.armlinux.org.uk>

On Thu, Jun 11, 2020 at 6:21 PM Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:

> If you increase the page size, then you need to increase the number
> of tables in a page, or suffer a huge amount of wasted memory taken
> for the page tables - going to an 8k page size means that the upper
> 4k of each page will not be used.  Going to 16k means the upper 12k
> won't be used.  And so on - as your software page size increases,
> the amount of memory wasted for each PTE table will increase
> unless you also increase the number of hardware 1st level entries
> pointing to each PTE page.  With 64k pages, 60k of each PTE page
> will remain unused.
>
> That isn't very efficient use of memory.

I think this could be addressed by using the full page to contain
PTEs by making PTRS_PER_PTE larger and PTRS_PER_PGD
smaller, but there is an even bigger problem in the added memory
usage and I/O overhead for basically everything else: in any
sparsely populated memory mapped file or anonymous mapping,
the memory usage grows with the page size as well.

I think Synology's vendor kernels for their NAS boxes have a
different hack to make large file systems work, by extending
the internal data types (I forgot which ones) to 64 bit. That is
probably more invasive to the generic kernel code, but should
be much more efficient and less invasive to ARM architecture
specific code.

Either way, I wonder what the intended use cases are. Is this
work mainly intended for

a) running Debian/Buildroot/Yocto/... with (close to) upstream
   kernels on older NAS boxes,
b) commercial products that use 32-bit SoCs in multi-disk
    NAS boxes with vendor upgrades to future kernels, or
c) commercial products using 64-bit SoCs but 32-bit kernels?

My feeling is that any commercial products that need this are
either stuck on old kernels already, or they have moved on
to 64-bit chips and are better off running a 64-bit kernel[1], so
a) seems like the main purpose, right?

       Arnd

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